This invention relates to a stencil printer for reading an original image through image input means such as a scanner, performing image processing for read original image data, perforating a heatsensitive stencil sheet, and printing (digital printer), a system using an electrophotographic technology for forming a latent image on a photosensitive body and transferring the image to paper (digital copier), or a method and a system for copying onto thermosensible paper, etc.
Generally, pseudo gradation processing using a dot area modulation method including a systematic dither method is used to represent light and shade of an image of multiple-valued image data read through original input means on an image output device that can output only binary data of "0" and "1."
The systematic dither method is as follows: M.times.N pixels are considered a minimum unit of gradation representation and an M.times.N threshold matrix corresponding thereto is prepared and is superimposed on the original image with the threshold matrix as a kind of mask for comparing the density of each pixel with its corresponding threshold value. If the image data is greater than the threshold value, "1" is output; if the image data is less than the threshold value, "0" is output. Upon completion of the M.times.N pixel processing, the threshold matrix is moved to the next M.times.N pixel position and the same processing is repeated for binarization.
In the pseudo gradation processing using the systematic dither method, gradation is not compatible with resolution. To provide sufficient gradation, the threshold matrix size needs to be enlarged, thus reproducibility of an area requiring resolution like characters worsens.
A method for solving the problem is described in Japanese Patent Application Laid-Open No. Sho 63-180273 as follows: Edge resemblance of an image is calculated and the area is divided into a character area and a photo area according to the calculation result. Binarization with a single threshold value is executed in the character area and binarization with the systematic dither method using a threshold matrix for outputting halftone dots is executed in the photo area. The two different processes are switched for use in both the areas, thereby providing pseudo gradation representation good in gradation and resolution.
In the method, however, the image is separated into the character area and the photo area according to the edge resemblance of the image and the two different binarization processes are switched for each area boundary, so that an output image becomes discontinuous, resulting in a sense of incongruity.